1. Field of the Invention
The present invention relates to a method of properly controlling the printing condition of a thermal print head incorporated into a thermal printer to print characters and the like in uniform print density.
2. Description of the Related Art
Generally, the thermal print head is formed by arranging a plurality of heating elements in a single or a plurality rows on a ceramic substrate, and power is supplied to the heating elements so as to make the heating elements generate heat selectively for printing. Since the substrate is heated by the heat generated by the heating elements, the heating element energizing time is varied by preceding condition correction and area condition correction according to the temperature of the substrate to avoid uneven printing.
Antecedent condition correction compensates a change in the temperature of the objective heating element, i.e., the heating element to be energized, attributable to a change in the ambient temperature, more specifically, a change in the temperature of the objective heating element itself and the temperature of a glaze layer of several ten micrometers in thickness heated by the heat generated by the objective heating element. Thus preceding condition correction corrects the energizing time according to the mode of energization of the objective heating element and the adjacent heating elements in the preceding printing cycles. Since the heating elements and the glaze layer have each a comparatively small heat capacity, the preceding condition correction may be made on the basis of the mode of energization of the heating elements in the several preceding printing cycles.
Area condition correction corrects the energizing time for energizing the objective heating element according to the position of the objective heating element on a printing line to prevent the difference in print density between the first character at the head of a line and the last character at the tail of the same line resulting from the gradual rise in the temperature of the substrate of the thermal print head from a temperature at the head of the line to that at the tail of the line when the heating elements are energized repeatedly for printing characters on the line. Since the heat capacity of the substrate is far greater than that of the heating elements, a correction for area condition correction must be determined on the basis of the mode of energization of the heating element in a time period preceding the energization of the heating element longer than a time period to be taken into consideration for preceding condition correction. Generally, energizing cycles repeated after the start of printing on a print line is counted and a correction for area condition correction is determined on the basis of the number of energizing cycles. Since the substrate has a comparatively large heat capacity, the correction for area condition correction is smaller than that for preceding condition correction.
Energizing time is determined for each heating element on the basis of corrections for preceding condition correction and area condition correction determined for each heating element. For example, the sum of a preceding condition correction time and an area condition correction time is added to a standard energizing time to determine an actual energizing time.
However, this conventional method of controlling the printing condition of a thermal print head, which determines a specific energizing time for each heating element, needs a timer for each heating element to set an energizing time for each heating element.
Although progressive miniaturization of heating elements for thermal print heads for improvement of print quality or progressive increase in the number of heating elements of the thermal print head for the enhancement of printing speed has been made in recent years, enhancement of printing speed and miniaturization of heating elements are incompatible with each other when the temperature of thermal print heads is controlled by the conventional method because conventional methods needs complicated calculations for each heating element.
In the conventional method of controlling the printing condition of a thermal print head, which needs a timer for each heating element, the number of timers increases with the increase of the number of the heating elements to increase the manufacturing cost of a controller for controlling the thermal print head. For example, although only twenty-four timers are necessary for controlling the temperature of a thermal print head having twenty-four heating elements, forty-eight timers, 128 timers and 160 timers are necessary for controlling thermal print heads respectively having forty-eight heating elements, 128 heating elements and 160 heating elements, respectively. Thus, the manufacturing cost of a controller for controlling the thermal print head increases with increase in the number of the heating elements of the thermal print head.